1. Field of the Invention
The present invention relates to a procedure for the determination of an absolute position and to a position measuring instrument for forming the procedure.
2. Discussion of Related Art
In many fields, to determine the position of two bodies moved relative to one another, absolute position measuring instruments are increasingly being used. Absolute position measuring instruments have the advantage over systems that measure purely incrementally that in every relative position, even after the energy supply has been interrupted, a correct position information can be output immediately.
The absolute position is embodied by a code, which is arranged in a plurality of tracks extending parallel to one another, for instance in the form of a gray code.
Arranging the position information in a single code track, with code elements in line with one another in the measurement direction, is especially space-saving. The code elements are disposed in a pseudo-random distribution one after another, so that a certain number of successive code elements each form one code pattern or bit pattern that unambiguously defines the absolute position as a code word. When the scanning instrument is shifted by a single code element, a new bit pattern is already formed, and over the entire measurement range to be detected in absolute form, a sequence of different code words is available. This kind of sequential code is known as a chain code or a pseudo-random code.
In position measuring instruments, partial soiling or contamination of the scale can for instance cause errors in the bit string that forms the code word, and these cause an erroneous determination of the absolute position.
Methods for detecting such an error have already been proposed. In European Patent Disclosure EP 0 789 226 B1, it is proposed to this end that at least three complete, different code patterns from the scale be scanned simultaneously with a scanner and code words be formed from that. The actual spacings of the code words are compared with the set-point spacings, and as a result, from the simultaneously detected code words, one code word recognized as correct is used to determine the instantaneous absolute position, and code words detected as erroneous are excluded from further processing.
This method enhances the reliability and operating safety of a position measuring instrument. A disadvantage, however, is that a code word is already rejected as erroneous if only a single bit of the bit string is erroneous. To attain high reliability and operating safety of the position measuring instrument, many code words must be scanned simultaneously.
In German Patent DE 195 06 019 C2, a method for error recognition is described. In it, it is assumed that upon a change of position, the new code word must match a code word of the previous measuring step or the next measuring step. Only if the new code word matches a neighboring code word is the new code word assigned an absolute position. An error routine for determining an error is also disclosed. In it, bits of the new code word are inverted successively and compared for a match with the corresponding bit of the neighboring code words. This routine is performed until a match of the new code word with a neighboring code word is found. The bits that have to be inverted for the match are found to be erroneous, so that malfunctioning detector elements can also be detected. By this method, only error checking is possible.